Some have wondered why I made the previous post in isolation to the cost of a transit commute. Although I did point out that a transit commute saves at least the San Francisco-bound Novato commuter a ridiculous amount of money, I did not examine the county-wide costs because I wanted to emphasize the benefits to living where you can walk or bike to work. Transit, both in my post and Mr. Money Moustache's, is sloppy seconds: it still costs more than walking or biking.
Another reason I left out transit is the lack of work-trip fare data. Golden Gate Transit (GGT), the local bus and ferry agency, aggregates all fares for each mode into a single average fare. In addition, GGT operates outside of Marin as a regional bus service in Sonoma and is just about the only way to get into Marin by transit. Finally, the Blue and Gold Ferry, separate from GGT, doesn't even share its data, so its fares are left out. Each of these factors pollutes the data and makes any analysis less accurate.

But what the hell, right? Let's call this a back-of-the-envelope calculation, the kind you make when proving a point at a party. Don't take this as accurate, but take it as a rough idea of something approximating the actual amount Marin residents spend on transit and how much they could save by relying more on transit.

Golden Gate Transit's average fare for all riders on its ferries and buses was $3.02 in fiscal year 2010. Taking this as our base, we find that the average transit commuter spends about $1,500 per year on commuting and $19,500 over a decade. Good deal, a savings of $29,600 over the average car commuter per decade.

Transit makes up only 7% of work trips by Marin residents despite the cost savings. Playing out our $3.02 average fare means commuters spend about $9.6 million per year on transit, or $124.8 million over a decade. This is in comparison to the $7 billion we spend to commute alone. If 1% of our car commuters switched to transit, that would be a savings of $6.8 million per year and $88.6 million per decade. If our transit riders switched to single-passenger car commuting, they'd spend $88 million more per year and lose $1.1 billion in wealth over a decade.

Mind you, this is for the average commuter. While a car commuter from Novato to San Francisco saves $11,000 per year by switching to transit, a car commuter going to Solano County probably won't find transit a feasible option at any price. But if changing homes to be closer to work or changing jobs to be closer to home isn't an option, you should reexamine the long-term costs of that car commute and see how much taking the bus might save. You might be surprised by what you find.